The present invention relates to an adaptive-directivity transmission apparatus and adaptive-directivity transmission method that are applied to, for example, a base station apparatus in a mobile communication system.
An example of conventional adaptive-directivity transmission techniques is explained. The adaptive-directivity transmission is a technique that is studied on the premise that mainly a base station apparatus transmits a signal to a mobile station such as a portable telephone in a mobile communication system. Specifically, the base station first estimates a direction of a place where the mobile station is present based on reception weight information obtained in a reception circuit such as an adaptive array antenna provided in the base station. Then, the base station generates a directivity toward the estimated direction, and transmits a signal to the mobile station. Since the base station thus performs the adaptive-directivity transmission, it is possible to improve a reception gain in the mobile station, andfurthertoreduce interference to other mobile stations.
When this technique is used, in the case of a Time Division Duplex (hereinafter referred to as TDD) system in which transmission and reception carrier frequencies used in communications are same, it is possible to use a reception weight as a transmission weight without modifying the reception weight.
However, in the case of a Frequency Division Duplex (hereinafter referred to as FDD) system in which transmission and reception carrier frequencies used in communications are different, antenna intervals defined by a wavelength are different between a reception carrier frequency and a transmission carrier frequency. Therefore, it is not possible to obtain a directivity to be needed using the reception weight as the transmission with no modification performed. Accordingly, it is necessary to perform a correction on the reception weight due to a difference between the reception and transmission frequencies.
As methods for correcting the difference between the reception and transmission frequencies, there are following methods:
{circle around (1)} Convert reception weights into transmission weights collectively using a predetermined conversion equation in the case where an arrival direction of a received signal is recognized in advance, or where an arrival direction of a received signal is estimated;
{circle around (2)} obtain a transmission weight for forming a transmission radiation pattern (hereinafter referred to as xe2x80x9ctransmission patternxe2x80x9d) with a directivity pointing only to an arrival direction of a desired signal, using a reception radiation pattern(hereinafter referred to as xe2x80x9creception patternxe2x80x9d) ; and
{circle around (3)} obtain a transmission weight for minimizing an error between a reception pattern and a transmission pattern using a predetermined calculation equation with a negative feedback control. Thus, in the adaptive-directivity transmission in a conventional FDD system, the transmission weight is obtained from the reception weight with the predetermined equation, and then the appropriate transmission pattern is achieved.
However, in the above-described method {circle around (3)} of collectively converting, in the case where the transmission pattern obtained with the transmission weights that are collectively converted is compared to the reception pattern, the patterns of a portion around a signal arrival direction are approximate in both radiation patterns, but a difference between the transmission pattern and the reception pattern becomes large in the portion away from the signal arrival direction as the portion goes away therefrom.
Further, in the above-described method {circle around (3)} of obtaining a transmission weight for minimizing an error between a reception pattern and a transmission pattern using a predetermined calculation equation, the approximation is poor in a direction where an antenna directive gain is extremely suppressed, called null point. Furthermore, since it is necessary to examine a pattern error for all directions (0xc2x0 to 360xc2x0) on the two-dimensional plane, this method requires a lot of time and calculation amounts by the time a value of the obtained transmission weight converges.
The null point is generated to suppress the interference signal, for example, when a base station receives the interference signal against a desired signal at the time the desired signal is received from mobile station A. Therefore, since it is considered that another base station B is present at the direction of the null point, in the case where the base station performs the adaptive-directivity transmission to mobile station A, if the base station suppresses a signal transmission level to the direction of the null point, the interference in mobile station B is thereby reduced. Accordingly, it is important, for the improvement of the system performance, to obtain the transmission pattern of which the null point is approximated to that in the reception pattern.
An object of the present invention is to provide an adaptive-directivity transmission apparatus and adaptive-directivity transmission method which can improve the approximation of a transmission pattern and reception pattern at a directive antenna gain suppressed direction, while being capable of reducing a time and calculation amount required by the time a value of transmission weight converges, in the case of using the method for obtaining a transmission weight that minimizes an error between the transmission pattern and reception pattern with a predetermined calculation equation.
To achieve the above object, the adaptive-directivity transmission apparatus according to the present invention subjects the error between the transmission pattern and reception pattern to logarithmic transformation, and obtains a transmission weight that minimizes the error subjected to the logarithmic transformation. The directive antenna gain is generally represented using dB as a unit. The directive antenna gain at a null point is normally smaller than the maximum value of the directive antenna gain by 30 dB to 40 dB. Otherwise, the gain at the null point is {fraction (1/1,000)} to {fraction (1/10,000)} the maximum value in the true value. Accordingly, since the error between the transmission pattern and reception pattern around the null point becomes an extremely small value,in the adaptive-directivity transmission apparatus according to the present invention the error is calculated using dB, not the true value. In the adaptive-directivity transmission apparatus according to the present invention it is thus possible to improve the approximation of the transmission pattern and reception pattern at the directive antenna gain suppressed direction.
Further, to achieve the above object, the adaptive-directivity transmission apparatus of the present invention limits a range in which the error between the transmission pattern and reception pattern is searched to a predetermined range. In the adaptive-directivity transmission apparatus according to the present invention it is thereby possible to reduce the time and calculation amount required by the time the value of transmission weight converges.